In dairy farming animals are milked and their milk is thereafter stored in a milk storage tank for collection on a regular time basis, e.g. every second day. In order to maintain the quality of the milk, it is cooled to approximately 4° C. as quickly as possible. It is necessary to be careful during cooling of the milk since the quality is deteriorated if the milk freezes.
At a dairy farm provided with an automatic milking system, the milk usually enters the milk storage tank in small amounts spread during the day and night, compared to a dairy farm without an automatic milking system, where all animals are milked together two or three times a day. The milk storage tank is usually equipped with a cooling device, which lowers the temperature of the milk to about 4° C. and maintains this temperature in a filled milk storage tank. When the milk storage tank only contains small amounts of milk, there is a considerable risk of cooling the milk too much since the cooling device, when cooling, operates at full capacity.
Usually the cooling of milk in a milk storage tank is controlled in response to the temperature of the milk in the tank. The milk temperature is usually measured on the outside of the tank due to hygienic requirements, and this results in a rather slow response when the temperature of the milk within the tank is changed. Such a sluggish temperature response gives rise to problems, in particular when the volume of stored milk is small. The temperature of a small milk volume is lowered rapidly at the risk of freezing the milk.
Furthermore, the milk storage tank is typically provided with a stirrer, which stirs the milk in the tank to obtain a uniform temperature of the milk. Such a stirrer is usually not able to stir the milk in the tank when the tank contains only small amounts of milk, which thus involves a further risk of freezing milk locally at the beginning of the filling of the tank with milk. If cooling, on the other hand, is omitted while there is only a small volume of milk in the milk storage tank, this milk is not cooled at once, and thus the quality of the milk is deteriorated.